z6 release notes and version update.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

doc/release-notes/release-notes-0.11.2.z6.md

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+Jack Grigg:
+      Equihash: Only compare the first n/(k+1) bits when sorting.
+      Randomise the nonce in the block header.
+      Clear mempool before using it for benchmark test, fix parameter name.
+      Fix memory leak in large tx benchmark.
+
+Sean Bowe:
+      Increase block size to 2MB and update performance test.
+      Make sigop limit `20000` just as in Bitcoin, ignoring our change to the blocksize limit.
+      Remove the mainnet checkpoints.
+      Fix performance test for block verification.
+      Make `validatelargetx` test more accurate.
+
+Taylor Hornby:
+      Add example mock test of CheckTransaction.
+
+aniemerg:
+      Suppress Libsnark Debugging Info.
+